The Justice Department yesterday said it would attempt to block a sale of Eastern Air Lines's landing and takeoff rights at National Airport to United Airlines, a move that appears to clear the way for a sale to Northwest Airlines.

United had bid $35.5 million and Northwest $35.25 million earlier this month for 67 Eastern slots and five gates at National being auctioned off by the court overseeing Eastern's bankruptcy. But the Justice Department yesterday said it would file an antitrust suit to stop the sale to United on the grounds that it would reduce competition at National.

U.S. Bankruptcy Judge Burton R. Lifland in New York is scheduled to rule today on the sale of Eastern's slots at National, each of which is the equivalent of one landing and takeoff.

Sources involved in the bankruptcy case said yesterday they were certain that Lifland would now award the access rights to Northwest. There is still a possibility that another airline could enter a bid for access to National, but that appeared unlikely.

Northwest was cautious in its reaction to the Justice Department decision. "We're very pleased," a Northwest spokeswoman said. But she cautioned that it was still up to the bankruptcy court. United Airlines said it was "disappointed" by the government decision, but gave no indication it would fight it in the courts.

Northwest said yesterday that if it does get the National slots, it will establish direct, nonstop service to eight Florida cities, including Miami and Tampa, that it now serves from other airports. In addition, an airline spokeswoman said, Northwest will probably start serving New York City and Philadelphia from National. Asked if the airline was considering a shuttle service to the New York area in light of the problems now facing Pan American Airways and the Trump Shuttle, the spokeswoman said, "I'm sure that's being considered, but no decisions have been made."

James F. Rill, the assistant attorney general in charge of the antitrust division, said the proposed sale to United would "lessen competition" between Washington and other cities because the airline is already the dominant carrier at Dulles International Airport and because the acquisition of the Eastern rights would give it control of 20 percent of the slots at National.

Rill said the sale to United would reduce the bulk of the competition at National to two airlines: United and USAir. USAir is by far the dominant carrier serving the Washington area, with 75 daily flights out of National, 155 daily flights at Baltimore-Washington International and nine daily flights at Dulles.

Rill said the department's particular concern over a sale to United is a reduction of service from Washington to cities in the Northeast such as Hartford, Buffalo and a number in Florida. He also noted that Northwest plans to create a hub in Atlanta and access to National is needed if the airline hoped to make the Atlanta operation viable.

With the disappearance of Eastern, the Atlanta market is now dominated by Delta Airlines, which has bid $135 million for 18 gates at Atlanta's Hartsfield Airport as well as other assets in the Atlanta facility. Delta has bid $5.4 million for nine slots at National.

Northwest currently serves Boston, Detroit, Memphis, Milwaukee and Minneapolis from the Washington area with 26 daily flights at National, and six each at Dulles and BWI.